Compress Raster Data File??

Have a raster file of elevation data for a region in the SW of England. It is 4000rows x 4000columns large, but the program I'm running cannot take more than 100 x 100. A) How do I first output a text file that only covers the area within the red catchment (as seen in the attached screenshot)? Catchment boundary is using a table of coordinates. B) How do I compress this data down to only 100x100? I know it will become more pixelated but that's the nature of program I'm using!

Also attached is a sample screenshot of the current format of the raster file I currently have..

Any quick replies will be much appreciated guys, really need some help on this. Cheers